Circuit Transistor Tester Schematics

This tester is intended to quickly check whether a transistor is functional or not and possibly also select two or more transistors with (approximately) equal gains. This is about the simplest conceivable test circuit, so don’t expect super accuracy. The circuit has been designed only to quickly carry out a brief check, when there is no time or equipment to carry out a thorough test. The operation is simple: in the position ‘battery test’ (S2 closed), the 10mA moving coil meter M1 in series with a 600 Ω resistor (R4 + R5) is connected to a 6 V battery. A current of 10mA will flow, resulting in full-scale deflection of the meter. When a transistor is being tested (S2 open, S3 in position 2 or 3) a current will flow through the base-emitter junction of the transistor under test, the value of which can be computed by dividing the voltage across R1 or R2 by its resistance.

With S3 in position 2 this will be (6 V – 0.6 V)/560 kΩ = approx. 10µA. If the transistor has a gain of 1000 it will cause a collector current (and therefore a meter current) of 10mA, causing full-scale deflection of the moving coil instrument. Therefore, the value indicated by the meter, when S3 is in position 2, has to be multiplied by a factor of 100 to obtain the gain of the transistor. In position 3 the base resistor is 10 times lower (R1 = 56 kΩ), so in this case the reading has to be multiplied by 10 to obtain the gain. It will be clear that position 2 of S3 is intended for high gains of up to 1000 and position 3 for gains of 0 to 100. The purpose of S1 is to reverse the polarity: the upper position drawn is for NPN transistors, the bottom for PNP types. If you have no moving coil instrument available, it is of course also possible to replace M1 with a digital meter.

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Circuit DTMF Receiver IC MT8870 Tester Schematics

Today, most telephone equipment use a DTMF receiver IC. One common DTMF receiver IC is the Motorola MT8870 that is widely used in electronic communications circuits. The MT8870 is an 18-pin IC. It is used in telephones and a variety of other applications. When a proper output is not obtained in projects using this IC, engineers or technicians need to test this IC separately. A quick testing of this IC could save a lot of time in research labs and manufacturing industries of communication instruments. Here’s a small and handy tester circuit for the DTMF IC. It can be assembled on a multipurpose PCB with an 18-pin IC base. One can also test the IC on a simple breadboard. For optimum working of telephone equipment, the DTMF receiver must be designed to recognise a valid tone pair greater than 40 ms in duration and to accept successive digit tone-pairs that are greater than 40 ms apart.

However, for other applications like remote controls and radio communications, the tone duration may differ due to noise considerations. Therefore, by adding an extra resistor and steering diode the tone duration can be set to different values. The circuit is configured in balanced-line mode. To reject common-mode noise signals, a balanced differential amplifier input is used. The circuit also provides an excellent bridging interface across a properly terminated telephone line. Transient protection may be achieved by splitting the input resistors and inserting zener diodes (ZD1 and ZD2) to achieve voltage clamping. This allows the transient energy to be dissipated in the resistors and diodes, and limits the maximum voltage that may appear at the inputs. Whenever you press any key on your local telephone keypad, the delayed steering (Std) output of the IC goes high on receiving the tone-pair, causing LED5 (connected to pin 15 of IC via resistor R15) to glow.

It will be high for a duration depending on the values of capacitor and resistors at pins 16 and 17. The optional circuit shown within dotted line is used for guard time adjustment. The LEDs connected via resistors R11 to R14 at pins 11 through 14, respectively, indicate the output of the IC. The tone-pair DTMF (dual-tone multi-frequency) generated by pressing the telephone button is converted into binary values internally in the IC. The binary values are indicated by glowing of LEDs at the output pins of the IC. LED1 represents the lowest significant bit (LSB) and LED4 represents the most significant bit (MSB). So, when you dial a number, say, 5, LED1 and LED3 will glow, which is equal to 0101. Similarly, for every other number dialled on your telephone, the corresponding LEDs will glow. Thus, a non-defective IC should indicate proper binary values corresponding to the decimal number pressed on your telephone keypad.

To test the DTMF IC 8870/KT3170, proceed as follows:

1. Connect local telephone and the circuit in parallel to the same telephone line.
2. Switch on S1. (Switch on auxiliary switch S2 only if keys A, B, C, and D are to be used.)
3. Now push key ‘*’ to generate DTMF tone.
4. Push any decimal key from the telephone keypad.
5. Observe the equivalent binary as shown in the table.
6. If the binary number implied by glowing of LED1 to LED4 is equivalent to the pressed key number (decimal/A, B, C, or D), the DTMF IC 8870 is correct.

Keys A, B, C, and D on the telephonekeypad are used for special signalling and are not available on standard pushbutton telephone keypads. Pin 5 of the IC is pulled down to ground through resistor R8. Switch on auxiliary switch S2. Now the high logic at pin 5 enables the detection of tones representing characters A, B, C, and D.

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Circuit Measuring Inductors Schematics

Often you find yourself in the position of needing to wind your own coil for a project, or maybe you come across an unmarked coil in the junk-box. How can you best find out its inductance? An oscilloscope is all you need. Construct a resonant circuit using the coil and a capacitor and connect it to a square wave generator (often part of the oscilloscope itself) Adjust the generator until you find the resonant frequency f.

When C is known (1000pF) the inductance L may be calculated from: L=1/(4π2.f2.C) If you are also interested how good the coil is i.e. what is its quality factor or Q, you can use the oscilloscope again. If the level of the damped oscillation drops to 0.37 (= 1/e) of the maximum after about 30 periods, then the Q factor of the coil is about 30. The Q factor should be measured at the intended operating frequency of the coil and with its intended capacitor. The coupling capacitor should by comparison be a much smaller value.

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Circuit High Side Current Measurements Schematics

It’s always a bit difficult to measure the current in the positive lead of a power supply, such as a battery charger. Fortunately, special ICs have been developed for this purpose in the last few years, such as the Burr-Brown INA138 and INA168. These ICs have special internal circuitry that allows their inputs to be connected directly to either end of a shunt resistor in the lead where the current is to be measured. The shunt is simply a low-value resistor, across which a voltage drop is measured whenever a current flows. This voltage is converted into an output current Io by the IC.

This current can be used directly, or it can be converted into a voltage by means of a load resistor RL. In the latter case, the ‘floating’ measurement voltage across the shunt is converted into a voltage with respect to earth, which is easy to use. The value of RL determines the gain. A value of 5 kΩ gives 1×, 10 kΩ gives 2×, 15 kΩ gives 3× and so on. It all works as follows. Just like any opamp, this IC tries to maintain the same potential on its internal plus and minus inputs. The minus input is connected to the left-hand end of the shunt resistor via a 5-kΩ resistor.

When a current flows through the shunt, this voltage is thus lower than the voltage on the plus side. However, the voltage on the plus input can be reduced by allowing a small supplementary current to flow through T1. The IC thus allows T1 to conduct just enough to achieve the necessary lower voltage on the plus input. The current that is needed for this is equal to Vshunt / 5 kΩ. This transistor current leaves the IC via the output to which RL is connected. If the value of RL is 5 kΩ, the resulting voltage is exactly the same as Vshunt. The IC is available in two versions.

The INA138 can handle voltages between 2.7 and 36 V, while the INA168 can work up to 60 V. The supply voltage on pin 5 may lie anywhere between these limits, regardless of the voltage on the inputs. This means that even with a supply voltage of only 5 V, you can make measurements with up to 60 V on the inputs! However, in most cases it is simplest to connect pin 5 directly to the voltage on pin 3. Bear in mind that the value of the supply voltage determines the maximum value of the output voltage. Also, don’t forget the internal base-emitter junction voltage of T1 (0.7 V), and the voltage drop across the shunt also has to be subtracted.

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Circuit Cheap And Cheerful Transistor Tester Schematics

By using a simple visual indicating system, this small transistor tester allows you to run a quick ‘go/non-go’ check on NPN as well as PNP transistors. If the device under test is a working NPN then the green LED (D1) will flash, while the red counterpart will flash for a functional PNP device. However if the transistor is shorted, both LEDs will flash, and an open-circuit device will cause the LEDs to remain off. The circuit is based on just one CD4011B quad NAND gate IC, four passive parts and two LEDs. The fourth gate in the IC is not used and its inputs should be grounded.

Alternatively, you may want to connect its inputs and output in parallel with IC1.C to increase its drive power to the transistor test circuit. IC1.A and IC1.B together with R2, R3 and C1 form an oscillator circuit that generates a low-frequency square wave at pin 4. This signal is applied to the emitter of the transistor under test as well as to inverter IC1.C. The inverted signal from IC1.C and the oscillator output then drive the test circuit (LEDs, device under test, R1) in such a away that the voltage across that part of the circuit is effectively reversed all the time.

For example, with an NPN transistor under test, when pin 10 is High and pin 4, Low, current flows through LED D1 and the forward biased transistor. However, no current will flow when pins 10 and 4 change states, since the transistor is then reverse-biased. The green LED, D1, will therefore flash at the rate determined by the oscillator. As you would expect to happen, a PNP transistor will be forward biased when pin 10 is Low and 4, High, enabling current to flow through the red LED in that case.

A supply rail of around 3 V (two series connected 1.5-V batteries) should be adequate. To prevent damage to the transistor under test, supply voltages higher than 4.5 V should not be used. Because the LED currents are effectively limited to a few mA by the output of IC1.C (also slightly dependent on the supply voltage), it is recommended to use high-efficiency devices for D1 and D2.

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Circuit Cable Analyser Schematics

Many constructors have various cables lying around and after a time do not know any longer what they are for or what their pin connections are. It is not always possible to check this with a multimeter. The analyzer may be of help in such a situation. In most cases, an analyser for checking cables with D9 and D25 connectors will suffice. The shape of the analyser will depend to a large extent on the type of cable to be checked. It may be made as a connector, as a bus, or as a feed-through cable. Since only standard components are needed, the cost is low.

Solder a resistor of 1 kΩ between pin 1 of the analyser and the case; one of 2 kΩ between pin 2 and the case, and so on, increasing the value of the resistor by 1 kΩ for each successive pin. When this is completed, connect the analyser to the cable to be tested and measure the resistance between pin 1 and the case. The value so obtained in kilohms is the number of the pin at the other end of the cable. The arrangement is shown in the diagram. If at all possible, use resistors in the E96 series, since these give best accuracy.

Since this design was completed, a reader has suggested a simple improvement to it, whereby the nine resistors are linked in series instead of in parallel. The great advantage of this simplification is that all nine resistors have the same value: 1 Ω or 1 kΩ. The test method remains the same: the value in Ω or kΩ measured on the multimeter is the nuber of the pin at the other end of the cable.

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Circuit Thyristor Tester Schematics

The circuit in the diagram is a very handy tool for rapidly checking all kinds of thyristor (SCR, triac, …). In case of a triac, all four quadrants are tested, which is done with S3, while in case of a standard thyristor, a positive power supply and trigger current need to be set, which is done with S1. The value of resistors R1 and R2 is chosen to obtain a current of about 28 mA, which is more than sufficient for most thyristors. The hold current is determined by R3, and is 125 mA, which is more than adequate to keep the thyristor in conduction after it has been triggered. Since D1 is a red, low-current LED, and D2 a green, low-current LED, it can be seen in a wink in which quadrant the thyristor conducts.

Testing is started with S2, and the circuit is reset with S4 after the test has been concluded. Three short lengths of circuit wire terminated into insulated crocodile clips on connector K1 will be found very convenient for linking any kind of thyristor to the circuit. Mind correct connections, though: in the case of a triac, MT1/A1 is linked to earth, the gate to S2 and MT2/A2 to R3; in the case of a standard thyristor, the anode is linked to R3, the cathode to earth, and the gate to S2. If, in a rare case the trigger current needs to be altered, this can be done by changing the value of resistors R1–R3 as appropriate. The trigger current may also be made variable by the use of a variable power supply. If that is done, make sure that the dissipation in the resistors is not exceeded.

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Circuit Liquid-Crystal Display (LCD) Tester Schematics

Liquid-crystal displays come in all sorts and sizes, and this applies also to their pinouts. In fact, many of these displays cannot be used properly without the manufacturers’ documentation. But, of course, this can never be found when it is needed, and a small tester to unravel the terminals may, therefore, be found very handy. A liquid-crystal display consists of two thin sheets of glass, the facing surfaces of which have been given thin conducting tracks. When the glass is looked through at right or near-right angles, these tracks cannot be seen. At certain viewing angles, they become visible, however.

The space between the sheets of glass is filled with a liquid that, stimulated by an electric voltage, alters the polarization of the incident light. In this way, segments may appear light or dark and give rise to the display of lines or shapes. A segment may be tested by applying an alternating voltage of a few volts across it. Note that the application of a direct voltage will damage the display irreversibly: the resulting current will remove the tracks. The alternating voltage should contain not even a tiny direct voltage component. An alternating current also removes part of the tracks when the current flows in one direction, but restores it when the current flows in the opposite direction.

The tester described here consists of a square-wave generator that produces an absolutely symmetrical alternating voltage without any d.c. component. Most logic oscillators are incapable of producing a squarewave signal: they generate rectangular waveforms whose duty cycle hovers around the 50%. The 4047 used in the tester has a binary scaler at its output that guarantees symmetry. The oscillator frequency is about 1 kHz. It may be powered from a 3–9 V source. Normally, this will be a battery, but a variable power supply has advantages. It shows at which voltage the display works satisfactorily and also that there is a clear relationship between the level of the voltage and the angle at which the display is clearly legible.

The tester draws a current not exceeding 1 mA. The test voltage must at all times be connected between the common terminal, that is, the back plane, and one of the segments. If it is not known which of the terminals is the back plane, connect one probe of the tester to a segment and the other successively to all the other terminals until the segment becomes visible. Note, however, that there are LCDs with more than one back plane. Therefore, if a segment does not become visible, investigate whether the display has a second back plane terminal.

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Circuit Infra-Red Remote Control Tester Schematics

This little circuit is invaluable for quick go/no-go testing of just about any remote control transmitting infra-red (IR) light. The tester is battery-powered, built from just a handful of commonly available and inexpensive parts, and fits in a compact enclosure. Schmitt trigger gate IC1f is used as a quasi-analogue amplifier with, unusually, an infra-red emitting diode (IRED) type LD274 acting as the sensor element. An R-C network, C1-R2, is used at the output of the gate because all IR remote controls transmit pulse bursts, and to prevent the output LED, D2, lighting constantly when day-light or another continuous source of IR light is detected.

Circuit diagram:



Cased project:
This creates a useful ‘quick test’ option: point the tester at direct daylight, and the indicator LED should light briefly. The sensitivity of the tester is such that IR light from remote control is detected at a distance of up to 50 cm. The circuit is designed for very low power consumption, drawing less than 1 mA from the battery when IR light is detected, and practically no current when no light is detected. Hence no on/off switch is required. The construction drawing shows how the tester may be ‘cased’ using a small ABS case from Conrad.

COMPONENTS LIST
Resistors:
R1,R2 = 10MW
Capacitor:
C1 = 10nF
Semiconductors:
D1 = LD274 (Siemens)
D2 = LED, 3mm, low-current
IC1 = 74HC14
Miscellaneous:
Bt1 = 3V Lithium cell with solder tags, e.g.type CR2045 (560 mAh)

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Circuit Three-State Continuity Tester Schematics

The continuity tester can distinguish between high-, medium-, and low-resistance connections. When there is a conductance between the inputs, which are linked to small probes, a current flows from the +9 V line to earth via R1 and R2. The consequent potential difference, p.d., across R2 is used to determine the transfer resistance. Operational amplifier IC1c amplifies the p.d. across R2 to a degree that is set with P1. A window comparator, IC1a and IC1b, likens the output of IC1c to the two levels set with potential divider R4–R6. Depending on the state of the outputs of the two comparators, three light-emitting diodes (LEDs) are driven via the gates and inverters contained in IC3 and IC2 respectively in such a way that they indicate the transfer resistance in three categories.

When the resistance is high, green diode D3 lights; when it is of medium value, yellow diode D2 lights, and when it is low, red diode D1 lights. The levels at which the diodes light is set with P1, but note that in any case the minimum value depends on the p.d. across R2. It is possible to reduce the value of the p.d. to enable lower transfer resistances to be detected, but this would mean an increase in the test current through R2. With values as specified, the circuit in its quiescent state draws a current of about 17 mA, but in operation each LED adds about 10 mA to this. The LM324 (IC1) may be operated from a single supply line: R1 prevents the voltage at the input from reaching the level of the supply line (which is not permissible). The supply voltage may be 5–18 V. The LEDs are driven directly by the inverters in the 4049 (IC2), which can switch currents of up to 20mA to earth.

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Circuit Infrared Remote Control Tester Schematics

As I was developing my IR Extender Circuit, I needed to find a way of measuring the relative intensities of different Infra red light sources. This circuit is the result of my research. I have used a photodiode, SFH2030 as an infra red sensor. A MOSFET opamp, CA3140 is used in the differential mode to amplify the pulses of current from the photodiode. LED1 is an ordinary coloured led which will light when IR radiation is being received. The output of the opamp, pin 6 may be connected to a multimeter set to read DC volts. Infra red remote control strengths can be compared by the meter reading, the higher the reading, the stronger the infra red light. I aimed different remote control at the sensor from about 1 meter away when comparing results. For every microamp of current through the photodiode, about 1 volt is produced at the output. A 741 or LF351 will not work in this circuit. Although I have used a 12 volt power supply, a 9 volt battery will also work here.

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Circuit Connection Tester Schematics

A low resistance ( 0.25 - 4 ohm) continuity tester for checking soldered joints and connections.

This simple circuit uses a 741 op-amp in differential mode as a continuity tester. The voltage difference between the non-inverting and inverting inputs is amplified by the full open loop gain of the op-amp. Ignore the 470k and the 10k control for the moment, and look at the input of the op-amp. If the resistors were perfectly matched, then the voltage difference would be zero and output zero. However the use of the 470k and 10k control allows a small potential difference to be applied across the op-amp inputs and upset the balance of the circuit. This is amplified causing the op-amp output to swing to full supply voltage and light the LED's.

Setting Up and Testing:

The probes should first be connected to a resistor of value between 0.22 ohm and 4ohm. The control is adjusted until the LED's just light with the resistance across the probes. The resistor should then be removed and probes short circuited, the LED's should go out. As the low resistance value is extremely low, it is important that the probes, (whether crocodile clips or needles etc) be kept clean, otherwise dirt can increase contact resistance and cause the circuit to mis-operate. The circuit should also work with a MOSFET type op-amp such as CA3130, CA3140, and JFET types, e.g. LF351. If the lED's will not extinguish then a 10k preset should be wired across the offset null terminals, pins 1 and 5, the wiper of the control being connected to the negative battery terminal.

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Circuit 12/24/48 V D.C. Tester Schematics

The present tester is intended primarily for testing the 24 V electrical circuits found on most pleasure craft. However, if the resistors are given different values, the circuit may, of course, be used for other voltage ranges. For 12 V, the value of the resistors should be 1.2 k?, and for 48 V, 4.7 k?. The tester should be connected to the +ve and –ve voltage rails with test clips or crocodile clips, whereupon the test probe is placed on the point to be tested. When the potential at the point is positive, the red LED lights; if it is negative, the green one does. If the supply is not connected to earth, the tester may be used as ground-leak tester. In this situation, one of the LEDs lights when the test probe touches a point at earth potential and there is a leakage.

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Circuit Improved Vibrating Battery Tester Schematics

Many blind and deaf-blind people use portable electronic devices to assist their everyday lives but it is difficult for them to test the batteries used in that equipment. Talking voltmeters are available for the blind but there is no commercially available equivalent usable by deaf-blind persons. This device enables blind, deaf-blind and sighted people to test batteries. It will test AAA, AA, C and D cells, as well as 9V "transistor" batteries. All rechargeable and non-rechargeable cell types are supported. The circuit needs no calibration. To use the tester, turn potentio-meter VR1 fully counter-clockwise and then connect the battery to be tested to the appropriate set of test terminals. If the battery has any usable charge, the pager motor in the tester will immediately vibrate.

VR1 is then slowly rotated in a clockwise direction just far enough to stop the vibration. The position of VR1 then indicates the loaded voltage of the battery on a scale of 1-1.5V (if the battery is connected to the 1.5V test terminals) or 6-9V (if the battery is connected to the 9V test terminals). A regulated +5.1V rail is generated from the battery under test with the aid of zener diode ZD1. For 9V tests, a 150O resistor limits the zener current, while diode D2 protects the circuit from reverse polarity battery connection. For 1.5V tests, a blocking oscillator formed by Q1, Q2 and L1 steps up the battery voltage before it is applied to the regulator. This configuration works reliably with inputs down to below 0.9V. The output of the oscillator is rectified by D1 and smoothed by the 33µF capacitor.

The circuit has to survive reverse connection of the battery under test. This creates a problem, because the LM393 cannot withstand a voltage more negative than -0.3V at its inputs. Diodes D1 and D2 indirectly protect the non-inverting inputs from negative voltages but series diodes cannot be used to protect the inverting inputs because of the unpredictable voltage drop they introduce. The solution used is to shunt negative voltages at the 1.5V test terminals with diode D3 in conjunction a 1kO resistor (R1). D3 limits the voltage at its cathode to about -0.7V, while resistors R2-R4 divide this by three to give no less than -0.23V at the inverting input (pin 2) of IC1a. When the battery is connected the right way around, D3 is reverse-biased and R1-R4 form a voltage divider that applies a quarter of the battery voltage to IC1a’s inverting input.

Similarly, D4 and R5-R10 protect the inverting input (pin 6) of IC1b from reverse-connected batteries at the 9V test terminals. However, in this case only 1/24th of the battery voltage appears at IC1b’s inverting input. Battery voltages in the range 1-1.5V at the 1.5V test terminals will therefore produce 0.25-0.375V at the inverting input of IC1a, while battery voltages in the range 6-9V at the 9V test terminals will produce 0.25-0.375V at the inverting input of IC1b. Potentiometer VR1 forms part of a voltage divider used to generate a comparison voltage that is variable over the same 0.25-0.375V range. This is applied to the non-inverting inputs of both IC1a and IC1b. When the sampled battery voltage exceeds this comparison voltage, the respective comparator output swings low, switching on Q3/Q4 to energise the pager motor.

The 68O resistor in the collector circuit of Q4 ensures that higher battery voltages do not overdrive the motor. When testing an earlier version of this circuit with batteries that have high internal impedance, it was found that when VR1 was advanced to the indicating point, the pager motor slowed down rather than switched off. This occurred due to a rebound in battery voltage at motor switch-off, which in turn caused the circuit to immediately switch the motor back on again. To counteract this effect, a small amount of positive feedback is applied around the comparators when the motor switches off. The feedback is disabled while the motor is running so that the indicating point of VR1 is not affected. This works as follows: when the motor is running, Q5 is conducting and D5 is reverse biased, so the comparison voltage at the non-inverting inputs of the comparators is not affected.

If the motor stops running, Q5 switches off and the 2.7MO resistor pulls the comparison voltage higher via D5 to ensure that the resulting battery voltage rebound does not restart the motor. Finally, diode D7 prevents reverse breakdown of Q4 in case of reverse battery connection at the 9V terminals. There is no need for a similar diode in the 1.5V part of the circuit because 1.5V is well below the reverse breakdown voltage of Q3. The prototype used "Magtrix" magnetic connectors on short flexible leads as the 1.5V test terminals. These allow the connection of AAA, AA, C and D cells but are arranged so that they cannot be brought closely together enough to connect 9V types. Unfortunately, magnetic connectors cannot be used for the 9V test terminals because some brands of 9V batteries have non-magnetic terminals. A conventional 9V battery snap can be used instead. For blind people, the knob on VR1 should be pointer-shaped (eg, DSE P-7102) so that the degree of rotation can be easily assessed by touch.

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Circuit 4-20mA Current Loop Tester Schematics

This design will interest technicians who work on pneumatically operated valves and other 4-20mA current loop controlled devices. Although 4-20mA signal injector/calibrators are available, this one is both cheap to build and easy to operate. When first powered up, the circuit sinks 4mA of current. If switch S1 is pressed, the current level slowly ramps up to 20mA, pauses and then ramps back to 4mA. This cycle will continue unless the switch is pressed again, whereby the output will lock to its current level. A further push of the switch resumes the prior cyclic operation. Output2 from the micro (IC1) is programmed to generate a pulse-width modulated (PWM) signal to drive the current sink transistor (Q1).


The digital PWM signal is converted to an analog voltage using a low-pass filter formed by the 1kω series resistor and a 4.7μF tantalum capacitor. By varying the PWM duty cycle and therefore the DC signal level out of the filter, the program can indirectly vary the current flow through the transistor. A 100 resistor in series with the emitter of Q1 converts the loop current to a small voltage, which is fed into the micro on input1. The program uses this feedback signal to zero in on the desired current level with the aid of the micro's analog-to-digital converter. Details of this can be seen in the accompanying program listing.

If the PICAXE senses an open circuit, it shuts down the output and goes into an alarm state, to alert the operator and protect the circuit under test. The switch can be pressed to reset operations to the start once the open circuit has been rectified. The LED flashes a code for various milestones, as follows: one flash at 4m and one flash to confirm a switch press two flashes at 12m when ramping up (for the first 5 cycles); three flashes at 20m and continued fast flash sequence for open-circuit alarm. For portable use, the circuit can be powered from two 9V batteries, whereas for bench testing, a 12V DC plugpack will suffice.

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Circuit Servo Tester Using A 4538 Schematics

There are times when a small servo tester for modeling comes in very useful. Everybody who regularly works with servos will know several instances when such a servo tester will come in handy. The function of a servo tester is to generate a pulsing signal where the width of the positive pulse can be varied between 1 and 2 ms. This pulse-width determines the position the servo should move to. The signal has to repeat itself continuously, with a frequency of about 40 to 60 Hz. These circuits often use an NE555 or one of its derivatives to generate the pulses. This time we have used a 4538 for variety. This IC contains two astable multi-vibrators. You can see from the circuit diagram that not many other components are required besides the 4538. The astable multi-vibrator in a 4538 can be started in two ways. When input I 0 (pin 5 or 11) is high, a rising edge on input I 1 (pin 4 or 12) is the start signal to generate a pulse.

The pulse-width at the output of IC1a is equal to (R1+P1)×C1. This means that when potentiometer P1 is turned to its minimum resistance, the pulse-width will be 10 k × 100 n = 1 ms. When P1 is set to maximum (10 k), the pulse-width becomes 20 k × 100 n = 2 ms. At the end of this pulse inverting output Q generates a rising edge. This edge triggers IC1.B, which then generates a pulse. The pulse-width here is 82 k × 220 n ˜ 18 ms. At the end of this pulse the Q output will also generate a rising edge. This in turn makes IC1.A generate a pulse again. This completes the circle. Depending on P1, the total period is between 19 and 20 ms. This corresponds to a frequency of about 50 to 53 Hz and is therefore well within the permitted frequency range.

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Circuit Meter Adaptor With Symmetrical Input Schematics

In contrast to an ordinary voltmeter, the input of an oscilloscope generally has one side (GND) connected to ground via the mains lead. In certain situations this can be very problematic. When the measuring probe is connected to a circuit that is also connected to ground, there is a chance that a short is introduced in the circuit. That the circuit, and hence the measurement, is affected by this is the least of your problems. If you were taking measurements from high current or high voltage (valve equipment) circuits, the out-come could be extremely dangerous! Fortunately it is not too difficult to get round this problem.

All you have to do is make the input to the oscilloscope float with respect to ground. The instrumentation amplifier shown here does that, and functions as an attenuator as well. The AD621 from Analog Devices amplifies the input by a factor of 10, and a switch at the input gives a choice of 3 ranges. A ‘GND’ position has also been included, to calibrate the zero setting of the oscilloscope. The maximum input voltage at any setting may never exceed 600 VAC. Make sure that R1 and R8 have a working voltage of at least 600 V. You could use two equal resistors connected in series for these, since 300 V types are more easily obtainable.

You should also make sure that all resistors have a tolerance of 1% or better. Other specifications for the AD621 are: with an amplification of 10 times the CMRR is 110 dB and the bandwidth is 800 kHz. If you can’t find the AD621 locally, the AD620 is a good alternative. However, the bandwidth is then limited to about 120 kHz. The circuit can be housed inside a metal case with a mains supply, but also works perfectly well when powered from two 9V batteries. The current consumption is only a few milliamps. You could also increase R9 to 10 k to reduce the power consumption a bit more.

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Circuit El Cheapo Cable Tester Schematics

Many cable testers have been published before, some quite complex, but here’s a cheap and simple alternative. It uses only a 9V battery, two mating connecators and a few resistors, as well as a multimeter for voltage measurements. Begin by soldering the leads of a 9V battery clip between the two pins furthest apart on one of the test connectors and then add a ladder of resistors between them for each of the other required pins. Any junk-box resistors will do but values between 1kΩ and 50kΩ are best. In the example shown, a five-core data cable terminated with D-9 connectors is to be tested. Connect 0V to pin 5, +9V to pin 1, and four resistors in between. Plug in a 9V battery and then probe the connector at the other end of the cable with a multimeter as indicated. Broken, shorted or incorrect connections are all quickly evident using this method.

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Circuit Four-Channel Oscilloscope Adaptor Schematics

This circuit enables you to display four signals simultaneously using only one channel of your oscilloscope. Essentially, it switches each of the inputs through to the output in turn, with some signal massaging in between. As shown, it is suitable only for low-frequency signal measurement and does not include over-voltage protection at the inputs. Each input circuit is identical, utilising one amplifier from a TL084 quad op amp package. Looking at channel 1, the input signal is attenuated by a factor of 10 by the 100kΩ and 10kΩ resistors before arriving at the non-inverting input of IC1a. A 15kΩ resistor in series with the op amp output along with a 10kΩ resistor to ground provide additional attenuation.

Vertical (voltage axis) adjustment is made with VR2, which sets the gain of the amplifier. This is used to calibrate or scale the displayed signal against the actual input voltage level. Using the values shown, the gain can be adjusted from unity to about a factor of 26. Note that the output of the op amp must be limited to ± 10V so that the voltage into the 4-channel multiplexer (IC2) does not exceed ± 5V. Therefore, with a gain of unity, the input voltage can range from ± 100V, whereas with a gain of 26, it must not exceed about ± 3.85V. VR1 applies a positive or negative DC offset to the output of the op amp. This can be used to compensate for op amp input offset voltage. It can also be used to shift the vertical position of the trace on-screen to provide multiple trace separation.

Note, however, that any offset will consume part of the available output swing and therefore limit signal measurement "headroom". Each op amp output is connected to IC2, a 4-channel analog multiplexer. The logic levels on "S0" & "S1" (pins 9 & 10) determine which input channel is connected to the "Z" output (pin 3). A square wave oscillator and divider circuit are used to toggle the "S0" and "S1" pins in sequence to first select channel 1 briefly, then channel 2 and so on. An LM6361 high-speed op amp (IC6) forms the heart of the oscillator. It operates at about 20kHz. Back-to-back zener diodes at the output clip the voltage to TTL levels, after which diode D1 passes the positive half-cycle to the input of one gate of a 74HC00 quad NAND device.

IC4a & IC4b "clean up" the signal before if is applied to the S0 input of IC2. A 74HC73 J-K flip-flip (IC5) divides the oscillator frequency by two. This is used to drive the "S1" input when in 4-channel mode. In 2-channel mode (switch S1 closed), one input (pin 12) of IC4d is pulled low, which effectively holds the "S1" input permanently low. Finally, a separate buffer circuit (IC3) is used to provide a trigger signal for the oscilloscope. This is needed because it would be difficult to trigger reliably on the main output as it switches rapidly between the four signal sources.

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Circuit Pipe Descaler Schematics

For many years now, magnetic (or electromagnetic) water descaler devices have been showing up on the shelves of Home Improvement and other DIY stores all over Europe. Despite the numerous studies completed on that subject, by manufacturers as well as by various consumer associations, none have been able to conclude on the efficiency of commercial pipe descalers in a decisive manner. Since electronic devices of this type are relatively expensive (especially when we discover what they are made of!), we decided to offer this project to our readers. For the price of a few tens of pounds, you will be able to evaluate the state of your own faucets, pots, and other pipes.

The device we’re offering as a project is identical to top-of-the-line items found on sale; in other words, it includes the bi-frequency option because it seemed that would be the best way to fight lime scale deposits. An initial astable oscillator, based on a traditional 555, labeled IC3, functions at around 10 kHz when the only capacitor C6 is operating; in other words, when T1 is blocked. The latter is controlled by another astable oscillator, based on IC1 this time, but which functions at about 1 Hz. When T1 is turned on by IC1, capacitor C4 is effectively in parallel with C6 which divides the frequency produced by IC3 by two, i.e. to about 5 kHz. In order to have high amplitude signals, the power supply operates with a mid-point transformer utilized in an unconventional way, with simple half-wave rectification.

The first half of the secondary delivers 15 VAC which, after being rectified, filtered and regulated by IC2, supply stable current of 12VDC to supply power to the oscillators. The entire secondary makes it possible to have available, after rectification, approximately 40VDC which is used to supply power to coils L1 and L2, wound around the pipe systems on which the assembly will work. To do that, IC3 is followed by high-voltage transistor T2 (a BF457 or equivalent) which chops this high voltage to 5 or 10 kHz frequency depending on the state of IC1. LED D3 lights up to signal that the power supply is present. Coils L1 and L2 are simple inductors made from insulated flexible wire, with about ten windings each.

They have to be wound around the pipes carrying the water to be ‘treated’ and are spaced about ten centimeters from each other. Neither the material of the pipe system, nor its diameter, should have any influence on the efficiency of the device. Paradoxically, these coils have one end in the air, which may surprise you as much as us but we indicated at the beginning of this article, that our goal with this project is not to explain the principle but rather to allow you to make the same device as those sold in stores, so that you can perform your own tests.

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